a) Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and more particularly to an embedded mesa semiconductor device such as a laser and a method of manufacturing the same.
b) Description of the Related Art
Optical communications technique utilizing semiconductor lasers and optical fibers has recently developed remarkably. Communications lines of main links have been replaced by optical fibers in place of electrical cables. Because of an expected increase of information processing amount of personal use, optical fibers are forecasted to be connected to houses.
FIG. 11 shows an example of a conventional embedded mesa semiconductor laser constituted by an n-type InP substrate 11, a mesa structure 20, an Fe doped semi-insulating InP embedding layer 16, a p-side electrode 17, and an n-side electrode 18.
On th e n-type InP substrate 11, an n-type InGaAsP guide layer 12, a non-doped InGaAsP active layer 13, a p-type InP clad layer 14, and a p-type InGaAsP contact layer 15 are grown in this order by metal organic vapor phase epitaxy (MOVPE) and selectively etched by dry etching to form the mesa structure 20. The surface of the n-type InP substrate 11 is lightly etched during the dry etching.
The Fe doped semi-insulating InP embedding layer 16 is formed covering the side walls of the mesa structure 20 and the exposed surface of the n-type InP substrate 11, by MOVPE using a source gas containing carbon tetrachloride (CCl.sub.4) or 1, 1, 1 trichloroethane (C.sub.2 H.sub.3 Cl.sub.3). The upper surface of the semi-insulating InP embedding layer 16 is made to be generally the same height as the upper surface of the mesa structure 20.
The p-side electrode 17 is formed covering the upper surface of the mesa structure 20 and the upper surface of the Fe doped semi-insulating InP embedding layer 16, and forms an ohmic contact with the p-type InGaAsP layer 15. The n-side electrode 18 is formed on the bottom of the n-type InP substrate and forms an ohmic contact with the n-type InP substrate 11.
As compared to wet etching, dry etching such as reactive ion etching has better controllability of etching amount, better reproductivity, and better uniformity of large area etching. MOVPE is capable of uniformly forming a layer on a large area of a substrate, and is suitable for mass production of semiconductor lasers by using dry etching.
By adding carbon tetrachloride (CCl.sub.4) or 1, 1, 1 trichloroethane (C.sub.2 H.sub.3 Cl.sub.3) to the source gas when the embedding layer is deposited, the surface of the substrate is more planarized.
The substrate surface planarized by depositing the embedding layer by adding carbon tetrachloride (CCl.sub.4) or 1, 1, 1 trichloroethane (C.sub.2 H.sub.3 Cl.sub.3) to the source gas, however, is not sufficient for practical use. As shown in FIG. 11, the surface of the embedding layer 16 swells up at the area near the mesa structure 20. The higher becomes the mesa structure, the more noticeable becomes this swell.
This step on the substrate surface leads to an insufficient patterning precision at the succeeding photolithography process and results in a lowered manufacturing yield.
If the amount of additive chlorine is increased or the growth temperature is increased in order to more planarize the substrate surface, the edges of the p-type InGaAsP contact layer 15 are etched during the growth of the embedding layer 16. The effective width of the p-type InGaAsP contact layer 15 is therefore reduced and the contact resistance increases.
Because of insufficient planarization, the thickness of the embedding layer is reduced at the area remote from the mesa structure. Electrostatic capacitance between the electrode formed on the thinned embedding layer and the substrate becomes large, and the high frequency characteristics are degraded.